1. Field of the Invention
The present invention relates to a switching circuit that switches a switching element connected to an inductance element with a pulse width modulation (PWM) signal, and an envelope signal amplifier including the switching circuit.
2. Related Background Art
In recent years, envelope elimination and restoration (EER) has been employed as one of the amplification schemes that amplify high-frequency modulated signals with power amplifiers, e.g., at base transceiver stations for cellular phones. With the EER scheme, an amplitude component (envelope) and a phase component are extracted from a modulated signal to be amplified, and a signal corresponding to the phase component is modulated through amplitude modulation with a signal corresponding to the amplitude component such that the amplitude of the modulated signal is at the same level as that of the original modulated signal.
More specifically, a voltage tracking the extracted envelope is used as a power voltage for a saturated amplifier which amplifies a signal corresponding to the phase component such that the amplitude of the amplified signal tracks the extracted envelope. The aforementioned voltage tracking the envelope can be obtained in a manner such that a modulated signal is amplified through power amplification with a detection signal (referred to as an envelope signal hereinafter) which is detected with the envelope, for example. A saturated amplifier is used for power amplification of the envelope signal for enhanced efficiency. For example, a switching element is switched using a PWM signal that is generated by modulating the pulse width of the envelope signal, and then the resulting PWM signal is integrated, thereby demodulating the envelope signal as a modulated signal.
In order to amplify the PWM signal, a D-class amplifier in which complementary switching elements are connected in a push-pull configuration, and an E-class amplifier in which a switching element is turned on when a voltage applied from an inductance element is zero are often used. Unfortunately, with the D-class amplifier, enhancement of the pressure resistance of the complementary switching elements in a balanced manner is technically difficult. Moreover, with the E-class amplifier, there is a case where a surge voltage greatly exceeds a power voltage depending on the design and operational conditions, the surge voltage being applied to the switching element from the inductance element when the switching element is turned off. As described above, application of the push-pull configuration and the single switching element to a high-frequency, high-power amplifier has limitations.
Furthermore, such a PWM signal includes a relatively low-frequency envelope signal component and a high-frequency PWM signal component; hence an amplifier to meet a broad frequency band is necessary for amplification of the PWM signals. In order to satisfy such conditions, a possible amplifier for the PWM signals is a distribution amplifier, for example, disclosed in Japanese Unexamined Patent Application Publication No. 2002-033627.